This invention relates to a printer having a heating head control circuit for driving a heating head which produces a thermosensitive recording on a thermosensitive recording medium. More specifically, the invention relates to a printer in which the heating head is divided into two side-by-side groups of heating elements which are energized and cooled in alternating fashion to make possible a reduction in the capacity of the heating head power supply.
A heating head is utilized to thermally color a thermosensitive recording medium such as thermosensitive paper, and forms desired patterns on the medium by means of the heat generated by the heating head. FIG. 1 illustrates an example of a circuit for controlling a heating head of this kind. The heating head 6' is equipped with heating elements 6.sub.1 ' through 6.sub.n ' arrayed in a single row or in zig-zag fashion on the reverse side of a substrate. In response to a command from a control unit 1' constituted by a microcomputer or the like, a drive current from a power supply 5' is selectively applied to those of the heating elements 6.sub.1 ' through 6.sub.n ' that correspond to the data to printed, whereby these heating elements are caused to emit heat and subject a thermosensitive paper to thermal energy in a dot-like pattern. The thermosensitive paper is thus caused to change color to form visible dots corresponding to the data. This recording process is performed across each line of the thermosensitive paper in line-by-line fashion. Specifically, after one line of data is recorded on the paper, the power supply 5' again supplies the drive current to those heating elements corresponding to the next line of data, and rollers feed the paper by one line so that the energized heating elements record the data on the next line.
The manner in which the energization of the heating head is controlled will now be described in greater detail. The control unit 1', assumed here to be composed of a microcomputer, supplies a shift register 2' with one line of print data PRD' in the form of a binary serial of "1"s and "0"s. The shift register 2', which successively shifts the print data PRD' in accordance with a shift clock, is adapted to store one line of the print data. The control unit 1' then produces a print enable signal PRE' and applies the signal to AND gates 3.sub.1 ' through 3.sub.n ', thereby opening the gates so that the print data PRD' stored in the shift register 2' may be applied as parallel data to respective switching transistors 4.sub.1 ' through 4.sub.n '. When the transistors 4.sub.1 ' through 4.sub.n ' are turned on, the respective heating elements 6.sub.1 ' through 6.sub.n ', connected in series with the transistors, are supplied with current from the power supply 5' and, hence, emit heat. Thus, the energized heating elements are selected in accordance with the print data PRD' received from the control unit 1' so that the heating head 6' records dots on the recording paper in a pattern decided by the selected heating elements.
Thus, in controlling the current feed to and the heating of the heating head in the conventional arrangement described above, all of the heating elements corresponding to one line of print data PRD are supplied en masse with current from the power supply 5 and, hence, emit heat simultaneously. Then, after a prescribed cooling period, heating elements corresponding to the next line of print data PRD are again selected and heated. According to this process, the recording of dots on the recording paper is performed by repeating a heating (H.sub.B) and cooling (C) cycle, shown by (9) in FIG. 3. It is therefore required that the power capacity of the power supply 5' be great enough to simultaneously energize all of the heating elements 6.sub.l ' . . . 6.sub.n ', and the power supply unit must be large in size. In addition, since the foregoing conventional arrangement relies upon repetition of a heating and cooling cycle, the power supply is idle during the cooling intervals, and the printer therefore, does not operate in an efficient manner.